The visibility of raster lines in a television display is a function of the size and brightness of the display. Large bright displays tend to appear coarse. Progressive scan displays have been proposed which minimize this problem by generating additional lines for the display. An example, wherein the added lines are replicas of the original scan lines, is described by R. A. Dischert in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983. In another form of progressive scan system, the "extra" lines for the display are obtained by interpolation of adjacent horizontal lines of the incoming video signal as described, for example, by K. H. Powers in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE SCAN ARTIFACTS which issued Aug. 23, 1983.
It has been recognized by Powers that a superior progressive scan image may be obtained in cases where there is no significant image motion by delaying the video signal by one field and interleaving time compressed lines of the incoming and field delayed video signals. In this way all 525 lines of an interlaced frame (NTSC assumed) are available for display during each field period thereby avoiding the loss of vertical resolution characteristic of conventional line interpolators. When motion occurs, however, the temporal difference (1/60 second for NTSC, 1/50 second for PAL) between the undelayed and the field delayed lines causes visible artifacts to be produced (e.g., double images, serrated edges, etc.).
Another approach to producing progressive scan conversion by field processing is described by Tanaka in Japanese Application Kokai (Laid Open) No. Sho 58-79379 entitled A TELEVISION RECEIVER published (laid open) on May 13, 1983. As in the field progressive scan system of Powers, a progressive scan image is produced by time compressing and interleaving lines of a current field with lines of a previous field obtained from a field memory. In the Tanaka system the storage requirement of the field memory is reduced by a factor of one-half by low pass filtering the video input signal before storing it in the field memory. The stored low frequency lines are recovered, time compressed and added to high frequency lines obtained by speeding-up (time compressing) and high-pass filtering the video input signal to generate interpolated interstitial or "extra" lines for display. The interpolated lines are then interleaved with lines of the time compressed non-field delayed video input signal to form the complete progressive scan output signal. In an embodiment of the Tanaka system the time compressed non-field delayed high frequency lines are also subjected to vertical interpolation after speed-up and before high pass filtering.